1. Area of the Art
The invention relates generally to fluid-tight dynamic seals between a reciprocating member and its housing. More particularly, the invention is directed to fluid-tight dynamic clearance seals and fluid-moving equipment utilizing such seals.
2. Description of the Prior Art
In many types of fluid-moving equipment, such as liquid pumps, slurry pumps, dry mixers, and numerous other devices, a sliding plunger, rod, piston, or another similar member, reciprocally moves inside a stationary bearing. Typically, fluid leakage around the moving member is prevented by utilizing sealing structures. The material of the sealing structure is required to have some resiliency and yet some degree of stiffness which will permit the moving member to slide back and forth through the axial opening of the sealing structure and yet be tight enough to prevent or at least minimize leakage of the liquid around the moving member.
One type of a conventional sealing structure is a mechanical face seal. Typically, the mechanical face seal consists of one seal ring rotating with the driving shaft and one stationary seal ring attached to the surrounding housing. The two seal rings are pressed towards each other by a biasing force which, in this way, prevents liquid from passing between them. For example, U.S. Pat. Nos. 3,282,235; 4,754,981; and 5,772,217 describe a seal with a spring for providing the biasing force. Usually, additional elastomeric components are required to seal each ring from the shaft or housing, correspondingly. Typically, a thin lubricating film is required between the seal surfaces to prevent their damage by dry friction. Nevertheless, with time, wear and vibrations cause the mating faces of the sealing rings to become scored, resulting in leakage of the process fluid. Environments where the process fluid is abrasive or contains a coagulant are particularly damaging to the conventional seals and require their frequent replacement.
A packed stuffing box is another example of a conventional seal for a moving member. This type of seal has been disclosed, for example, in U.S. Pat. Nos. 3,659,862 and 5,333,883. Generally, the packing is sufficiently compressed to limit the passage of fluid through the packing, but not so compressed as to create excess friction between the packing and the moving member. Pressure is generally maintained on the packing by manually tightening a gland on the stuffing box until the point where leakage through the packing is minimized, yet before the point where friction between the packing and the shaft creates overheating of the packing. Such a configuration operates on the principle of controlled leakage to the atmosphere rather than zero leakage. However, this requires frequent adjustment, and over tightening results in excess friction and heat buildup, excessive wear to the packing, and possibly even damage to the moving member. Even when the pressure on the packing is properly regulated, the pressure necessary to minimize the passage of fluid through the packing causes relatively high friction between the packing and the shaft. As a result, the packing wears out quickly and requires a frequent replacement.
Finally, referring to FIG. 1, a combination of an elastomeric O-ring 1 with a spring preloaded polymer seal 2 has been utilized in the past to create a dynamic seal between a moving member, such as a piston 3, and a housing structure. For example, Beckman Instruments (Fullerton, Calif.) uses this arrangement in the Access Immunoassay Analyzer. Typically, the housing structure includes a casing 4 and a piston-supporting bearing 5. The polymer seal 2 prevents the fluid from flowing between the piston 3 and the bearing 5, while the O-ring 1 seals off the housing. A preloading spring 6 forces the polymer seal 2 to squeeze directly on the piston 3 to accomplish the seal, thus resulting in wear of the polymer. The speed of the moving member and the fluid being sealed determines the required frequency of the polymer seal replacement.
Apparently, the sealing structures of prior art do not provide reliable and long-lasting seals between moving members and their housing. The conventional seals undergo a lot of wear during normal operation and have to be replaced frequently. The necessity to replace seals makes prophylactic maintenance of the equipment more laborious and increases its maintenance costs.